1. Field of the Invention
The present invention relates to an initial voltage establishing circuit and, more particularly, to an initial voltage establishing circuit for being applied in a switching voltage converter.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional switching voltage converter 10. The switching voltage converter 10 belongs to the step-up type, which converts a lower input voltage Vin into a higher output voltage Vout. An inductor L is coupled between the input voltage Vin and the switch node SN. A high-side switch SH is coupled between a switch node SN and an output terminal O, while a low-side switch SL is coupled between the switch node SN and a ground potential. Furthermore, an output capacitor Co is coupled to the output terminal O so as to filter ripples of the output voltage Vout. In the example shown in FIG. 1, the high-side switch SH is implemented by a PMOS transistor while the low-side switch SL is implemented by an NMOS transistor. A switch control circuit 11 applies a switch control signal CS to a driving circuit 12 for generating a high-side driving signal PH and a low-side driving signal PL. The high-side driving signal PH is applied to turn ON/OFF the high-side switch SH, while the low-side driving signal PL is applied to turn ON/OFF the low-side switch SL.
More specifically, the switch control circuit 11 adjusts the duty cycle of the switch control signal CS in response to the feedback of the output voltage Vout, thereby regulating the output voltage Vout at a target value. The switch control circuit 11 has a feedback circuit 13, an error amplifier 14, a PWM comparator 15, a reference voltage generating circuit 16, and an oscillating signal generating circuit 17, coupled together as shown in the figure. When the output voltage Vout is lower than the target value, the switch control signal CS output from the switch control circuit 11 provides a larger duty cycle to raise the output voltage Vout. When the output voltage Vout is higher than the target value, the switch control signal CS output from the switch control circuit 11 provides a smaller duty cycle to reduce the output voltage Vout.
At the beginning of activating the switching voltage converter 10, the duty cycle of the switch control signal CS output from the switch control circuit 11 is expanded to the maximum because the output voltage Vout at the output terminal O is at zero potential and therefore has a maximum difference from the target value. Such maximum duty cycle causes the low-side switch SL to be turned ON for a very long time, resulting in producing a huge inrush current to damage the circuit.